Generally, resins used in applications that require transparency include mono-component materials having a single refractive index. However, the use of mono-component materials is limited in various complex applications. Recently, many attempts have been made to prepare materials having transparency while maintaining desirable inherent characteristics of various materials.
In particular, studies on materials having reinforced impact strength and rigidity have been undertaken by mixing a conjugated diene-based polymer resin and an acrylic or a styrene polymer resin. However, since the difference between refractive indices of the conjugated diene-based polymer resin and the acrylic or the styrene polymer resin are significantly large, there is a need for a transparent thermoplastic resin having the same high level of transparency as that of monopolymer to offset the difference between the refractive indices. Furthermore, it is still difficult to satisfy good transparency and improved properties such as impact strength, flowability, and chemical resistance.
In the case of preparing transparent ABS resin, various methods in which acrylic resins are copolymerized with an opaque ABS resin are known. For example, U.S. Pat. No. 4,113,798 is directed to a method for making a transparent ABS resin by melt-extruding a graft-ABS resin and a SAN resin prepared by using a batch process. However, since an excess amount of modifier is added to prepare an initial resin, the method of melt-extruding the graft-ABS resin and SAN resin prepared by using a batch process has a disadvantage in that a desirable color in the final product cannot be obtained, thereby limiting its application.
Another method for making a transparent ABS resin uses a single continuous process. This process, however, is limited by the limited rubbery polymers than can be used due to the equipment used to carry out the continuous process. The equipment also limits the use of additives such as lubricants, depending on the amount and types of additives used. Therefore, it is very difficult to obtain both good impact strength and flowability by this method. Furthermore, the method is not suitable for mass production.
To overcome the above problems, U.S. Pat. Nos. 5,079,296 and 5,252,664 disclose that transparency can be obtained by mixing a block copolymer having rubber property and a resin having good rigidity for imparting transparency, and adding a small amount of rubbery particles to increase insufficient impact strength. However, it is still difficult to obtain sufficient impact strength merely with block copolymers having conventional rubber property, and the rubbery particles can lower transparency.
U.S. Pat. Nos. 5,237,007, 5,290,862, 5,344,878, and 6,734,247 disclose that transparency as well as desirable mechanical properties can be obtained by using two different rubbery copolymers having different contents of rubber for improving impact strength and also by using a copolymer having good rigidity for reinforcing insufficient rigidity. However, these methods have a disadvantage in that heat resistance as well as chemical resistance decreases due to large contents of rubber.